Use the macOS system local dictionary

**What problem does this solve?**
It would be nice if Harper could add custom words to the macOS system local dictionary so that custom spellings are synchronized with the system spell checker.

**Proposed Solution**
Can Harper simply use a symlink to `~/Library/Group Containers/group.com.apple.AppleSpell/Library/Spelling/LocalDictionary`?

I have two macOS system wordlists, and the first one seems more active:

- `~/Library/Group Containers/group.com.apple.AppleSpell/Library/Spelling/LocalDictionary`
- `~/Library/Spelling/LocalDictionary`

**Component**
- [X] Core engine
- [ ] Plugin/Extension
- [ ] Other: _____

**Additional Context**
Alternatively, if you want to keep Harper's custom wordlist separate during the onboarding process, you can prompt the user to import their existing custom words into Harper. Or, just tell them they can do `cat ~/Library/Spelling/LocalDictionary` to grab the existing list.

Start by locating Harper's core-engine custom wordlist handling and compare it with the two macOS LocalDictionary paths named in the issue. Determine which path is active and whether synchronization or import is the intended behavior. Done means Harper's custom words and the macOS system spell checker can use the agreed wordlist without losing existing entries.

Written by the indexing model from the issue text.